'Heterostyly' refers to those situations when a plant species has two or more different positional arrangements of its anthers and stigma(s), a genetic polymorphism providing a physical mechanism to promote outbreeding - which natural selection has determined is generally a 'good thing'.

This page currently deals with one such species, the Cowslip (Primula veris), a plant of pastures and short turf on well-drained, often lime-rich soils, still locally common in England but rarer and mainly coastal here in Scotland, though sometimes supplemented by misguided local planting or inappropriate use of 'wildflower' seed. It has suffered local declines and extinctions due to habitat loss and we have just one surviving population here in the Paisley area, native or long-established on a railway embankment.

The Cowslip has two basic arrangements of anthers and stigma:

'pin' plants, in which the style (ending in the stigma) is the length of the corolla-tube, i.e. the stigma is exposed at the mouth of the tube, and in which the anthers are fused to the walls of the corolla-tube, about half-way down. (Looking into the flower, the stigma looks like the head of a pin.)

'thrum' plants, in which the anthers are positioned at the mouth of the corolla-tube and in which the style is only half the length of the tube.

Consequently, a pollinating insect landing on the platform provided by the flower will insert its proboscis down the corolla-tube to get to the nectar at the base, and in doing so will pick up pollen in one of two positions. Thus pollen from a thrum plant is likely to be transferred to the stigma of a pin plant, and that from a pin plant transferred to a thrum. As these are only the two arrangements, this is an example of distyly.

(It might be noted that the pollen of a thrum plant could drop down the corolla-tube onto its own stigma. In fact the stigma is mature and receptive to pollen some 2-3 days before its own anthers dehisce (i.e. the flowers are protogynous) and as described below, thrum pollen is mechanically unable to fertilise a thrum flower.)

This genetic polymorphism is controlled by a super-gene with three major components:

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style length:

allele G codes for a short style, and also shorter papillae on the stigma surface, according to Briggs & Walters (1997). The recessive allele g codes for a long style and longer stigmatic papillae.

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pollen size:

allele P codes for thrum pollen, whereas the recessive allele p codes for pin pollen, which is only about two-thirds of the diameter of thrum pollen (note measurements on photomicrograph below).

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anther position:

allele A codes for anthers in the thrum position at the mouth of the corolla-tube; the recessive allele a codes for anthers positioned half-way down the tube.

It can be seen in each case that the dominant allele codes for 'thrum' characters and the recessive allele for 'pin' characters. Since thrum × thrum crosses cannot occur (or rarely so in the Cowslip, not at all in the related Primrose), thrums are heterozygous GPA/gpa, whereas pins are homozygous recessives, gpa/gpa.

Pollen of Primula veris, showing difference in size between pin and thrum pollen

The different pollen sizes restrict the possibilties of thrum × thrum and pin × pin crosses.

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Pin pollen, with its substantially lower amount of food reserves, is ill equipped to send its germ tube down the length of a pin style. Pin × pin crosses can occur, but at low frequency.

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Thrum pollen has the food reserves to send a germ tube down a pin style and so more than enough to cope with the shorter thrum styles. However, the larger diameter thrum germ tube is unable to penetrate the surface of a thrum stigma, though it can penetrate pin stigmas. Consequently, thrum × thrum pollination is mechanically impossible (or mostly so in the Cowslip, always so in the Primrose).

Excellent scanning electron photomicrographs showing stylar papillae, pollen (much more surface detail) and successful and unsuccessful pollinations are on the University of Hamburg 'Botany online' site here.

So successful pollinations are thrum × pin and pin × thrum (i.e. the reciprocal crosses with either thrums or pins as the pollen parent). Pin plants will produce gpa gametes only, while thrum plants will produce equal numbers of GPA and gpa gametes after meiosis. This is the equivalent of a Mendelian backcross, producing heterozygotes (thrums) and recessive homozygotes (pins) in equal numbers. However, as a small number of pin × pin crosses are successful, there is normally a small excess of pins over thrums in a population (certainly so in Primrose (P. vulgaris) populations, presumably also in Cowslip populations).

CONCLUDING REMARKS

The purpose of this page is to provide illustrations of heterostyly, not to duplicate accounts in text-books. Consequently there is no account here of the advantages of outbreeding, beyond mentioning the generally greater 'fitness' of heterozygotes (avoiding inbreeding depression, the possibilities of dominance modification, etc.). It should be noted that natural selection is reactive and can never plan for the future, so while maintenance of variability in a population may enhance its long-term chances of survival, this normally has little directly to do with the chances of a given individual passing on its genes to the next generation.

Rare crossings-over within the super-gene can produce other combinations of characters, notably in the case of the 'long-homostyle' recognised and studied in the Primrose (P. vulgaris), by J.L Crosby. In this variant, the 'pin' style and stigma are combined with the 'thrum' anther position and 'thrum' pollen. Consequently the long-homostyle can self-fertilise and may locally have a selective advantage over pins and thrums where inbreeding depression has not become a factor, where the genetically impoverished population is suited to its environment (as in two woods in S.W. England) and where the exposure of anthers and stigma together to slug damage is not a major issue. Understanding of the situation has been complicated by misidentifications and incorrect claims of homostyles; a useful and reliable account was given by Sheppard (1975).

Our British native species of primrose, i.e. P. veris, P. vulgaris, P. elatior and P. farinosa, are heterostylous, as are other members of the family Primulaceae here. It can be concluded that heterostyly is a clear advantage in these plants. The exception is the Scots Primrose, P. scotica, which is always homostylous and is self-fertile. Although regarded as an endemic species on our north coast, it is closely related to other sub-arctic plants undoubtedly derived from a common ancestor, in which guaranteed seed production in northern latitudes (despite unreliability of insect pollinators) has proved more important than loss of heterozygosity. I'll add a picture sometime.